A serious contemporary problem is the accidental spilling or intentional discarding of toxic or environmentally hazardous materials comprising both organic and inorganic contaminants. These injurious materials may be liquid or solid, and may be at the ground surface or buried. When such a situation occurs, the land affected can be permanently destroyed for productive use, or at least until an expensive clean-up process is accomplished. In addition to the effect on the soil, such contaminating wastes often find their way into streams, rivers and municipal water supplies, causing untold damage and potential sickness. Such hazardous waste materials include, by way of example, petroleum products, chemicals, mine tailings, sludges, and low-level radioactive materials whether exposed or contained in metal drums or underground storage tanks.
The sort of clean up operations which have been done in the past have involved excavating the contaminated soils or objects, and transporting them to a treatment facility or to a safe storage site. Alternatively, means of remediating contaminated soil in-situ have been outlined in U.S. Pat. No. 4,376,598 to Brouns et al., for "In Situ Vitrification of Soil". The '598 patent teaches the insertion of two or more conductive graphite electrodes into the soil, providing a conductive electrical path on the surface of the soil, and generating a current through the electrical path to heat the surrounding soil to its melting temperature. Once a melted soil stream has been established between electrodes, the molten soil provides the conductive path. However, this method has several drawbacks; e.g., soil melting must begin at the ground surface and proceed downward, it cannot operate at deep depths or at selective depths, the continuity of the soil electrical path is not reliable, and restarting the current flow once the melt has moved below ground surface level is quite difficult. In addition, since processing temperatures are below 2000.degree. C., soil additives may be necessary to lower the melting point of particular soils, and the method does not work well if there is too much metal or moisture in the soil.
The invention disclosed herein recognizes that there exists a relatively new technology which may be employed in the remediation of all contaminated soils or buried materials at any depth by the process of pyrolysis, melting and vitrification of waste materials using large quantities of very high temperature heat energy. The basic tool used in this technology is the plasma arc torch. Plasma arc torches can routinely operate at energy levels ranging from 100 kw to 10 MW. Plasma torches produce temperatures of 4000.degree. C. to 7000.degree. C. in the range of 85-93% electric to heat energy conversion efficiency. For purposes of comparison, the highest temperature attainable by fuel combustion sources or the aforementioned graphite electrode process is in the vicinity of 2700.degree. C.
A plasma arc torch operates by causing a high energy electric arc to form a stream of plasma, or ionized gas, thus generating large amounts of heat energy. There are many types of plasma torches, but all torches generally fall into one of two basic categories according to the arc configuration relative to the torch electrodes, i.e., transferred arc type and non-transferred arc type. The arc of a transferred arc torch is formed by and jumps from a single electrode on the torch, through the plasma gas, and to an external electrode which is connected to the opposite electrical terminal. The arc of a non-transferred arc torch is formed by and jumps from one electrode on the torch along the plasma gas and back to another electrode on the torch.
In the plasma arc torch, the heat energy produced is proportional to the length of the arc, assuming the type of plasma gas and applied electrical current both remain constant.
Since the present invention makes use of a plasma arc torch, reference is next made to U.S. Pat. No. 4,067,390 granted to the present inventors for "Apparatus And Method For The Recovery Of Fuel Products From Subterranean Deposits Of Carbonaceous Matter Using A Plasma Arc" which patent teaches the use of a plasma arc torch to gasify or to liquify underground deposits of coal, oil, oil shale, tar sands and other carbonaceous materials. The teachings of the '390 patent are incorporated hereby by reference.
It is a major objective of the invention to afford an efficient and environmentally safe system for the in-situ remediation of soils containing hazardous materials and other buried contaminants such as metal drums of waste matter, underground storage tanks and other toxic deposits.
It is a further objective of the invention to prevent the leaching and spread of contaminated material to the surrounding soil medium and to the subterranean water system.
Additional objectives of the invention will become apparent from the disclosure which follows.